/*
 * Copyright 2014 The Netty Project
 *
 * The Netty Project licenses this file to you under the Apache License,
 * version 2.0 (the "License"); you may not use this file except in compliance
 * with the License. You may obtain a copy of the License at:
 *
 *   http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
 * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
 * License for the specific language governing permissions and limitations
 * under the License.
 */
package io.netty.channel.epoll;

import io.netty.channel.unix.FileDescriptor;
import io.netty.util.internal.PlatformDependent;

/**
 * Tells if <a href=
 * "http://netty.io/wiki/native-transports.html">{@code netty-transport-native-epoll}</a>
 * is supported.
 */
public final class Epoll
{

    private static final Throwable UNAVAILABILITY_CAUSE;

    static
    {
        Throwable cause = null;
        FileDescriptor epollFd = null;
        FileDescriptor eventFd = null;
        try
        {
            epollFd = Native.newEpollCreate();
            eventFd = Native.newEventFd();
        }
        catch (Throwable t)
        {
            cause = t;
        }
        finally
        {
            if (epollFd != null)
            {
                try
                {
                    epollFd.close();
                }
                catch (Exception ignore)
                {
                    // ignore
                }
            }
            if (eventFd != null)
            {
                try
                {
                    eventFd.close();
                }
                catch (Exception ignore)
                {
                    // ignore
                }
            }
        }

        if (cause != null)
        {
            UNAVAILABILITY_CAUSE = cause;
        }
        else
        {
            UNAVAILABILITY_CAUSE = PlatformDependent.hasUnsafe() ? null
                    : new IllegalStateException(
                            "sun.misc.Unsafe not available");
        }
    }

    /**
     * Returns {@code true} if and only if the <a href=
     * "http://netty.io/wiki/native-transports.html">{@code netty-transport-native-epoll}</a>
     * is available.
     */
    public static boolean isAvailable()
    {
        return UNAVAILABILITY_CAUSE == null;
    }

    /**
     * Ensure that <a href=
     * "http://netty.io/wiki/native-transports.html">{@code netty-transport-native-epoll}</a>
     * is available.
     *
     * @throws UnsatisfiedLinkError if unavailable
     */
    public static void ensureAvailability()
    {
        if (UNAVAILABILITY_CAUSE != null)
        {
            throw (Error) new UnsatisfiedLinkError(
                    "failed to load the required native library")
                            .initCause(UNAVAILABILITY_CAUSE);
        }
    }

    /**
     * Returns the cause of unavailability of <a href=
     * "http://netty.io/wiki/native-transports.html">{@code netty-transport-native-epoll}</a>.
     *
     * @return the cause if unavailable. {@code null} if available.
     */
    public static Throwable unavailabilityCause()
    {
        return UNAVAILABILITY_CAUSE;
    }

    private Epoll()
    {
    }
}
